As an image display device such as a liquid crystal display or an organic EL display, a thin film transistor array (hereinafter, called a “TFT array”) using an organic semiconductor is used. Such a TFT array has a circuit structure formed by lining up a plurality of organic TFT elements in a matrix in association with pixels of an image display device. Here, in a case where a short circuit or a disconnection of a gate line G or a signal line S, or a defect due to a defective organic semiconductor thin film occurs, a corresponding organic TFT element does not operate normally, and is set to be in a state of so-called pixel omission in which pixels do not emit light. In addition, in a case where a variation in output properties or response speed is present in each TFT element constituting the TFT array, stable moving image display is not available.
Consequently, it is necessary to inspect a disconnection defect of the TFT array, or a variation in the output properties or response speed of each TFT element. As such an inspection method, a method of electrically measuring each element, an imaging method using infrared thermography, or the like has been known.
For example, Patent Documents 1 and 2 disclose an inspection method in which in an field emission display (FED) or liquid crystal display (LCD) panel, a signal line S is grounded, an appropriate direct-current voltage is supplied to a gate line G, and an image thereof is captured using an infrared camera. In a case where the signal line S and the gate line G are short-circuited to each other, infrared rays are radiated due to heat generation in the corresponding portion. Therefore, capturing the image thereof, using an infrared camera, allows to detect a radiant point, that is, a short circuit position.
In addition, a method is also considered which is intended to detect the light-emitting state of each TFT element, and to inspect not only a disconnection defect of the TFT array but also output properties or the like of each TFT element.
For example, Non-Patent Documents 1 and 2 describe that in an organic semiconductor thin film which provides a channel layer of an organic TFT element, light transmittance and optical reflectance change very slightly in a state where carriers are accumulated with application of a gate voltage, and a state where carriers are depleted without applying the gate voltage, and the amount of change is proportional to the amount of carriers accumulated, that is, an output current. It is considered that using such a phenomenon allows not only to detect a disconnection defect in the TFT array, but also to evaluate a variation in the output properties or response speed of each TFT element.